1. Field of the Invention
This invention relates to an electrophotographic type of image forming apparatus, such as a laser printer, photocopying machine, or FAX machine. More specifically, this invention relates to an image forming apparatus in which a charging member is brought into contact with a rotating photosensitive body in order to charge the surface of the photosensitive body.
2. Description of the Prior Art
An electrophotographic type of image forming apparatus, such as a photocopying machine, is known in which an image is formed under the following process.
The surface of a drum-like or belt-like photosensitive body is evenly charged by a charger and then exposed by an exposing device so as to produce an electrostatic latent image thereon. After that, a toner is caused to adhere to the latent image on the surface of the photosensitive body by a developing roller disposed in a developing device so as to make the latent image visible. The image-visualized toner is then transferred to the surface of transfer paper supplied from a paper supplying unit by a transfer device. The toner which has been transferred to the surface of the transfer paper is fixed by a fixing device, and then the transfer paper is discharged. Finally, a toner left, if any, on the surface of the photosensitive body is removed by a cleaning device.
In this type of image forming apparatus, a corona discharge method is adopted in order to evenly charge the surface of the photosensitive body. According to this method, the surface of the photosensitive body is charged due to ionization of the surrounding air.
However, the corona discharge method has a problem in that a great quantity of ozone is produced when the surface of the photosensitive body is charged. The ozone has the characteristic that when a minus discharge is made, much more ozone is produced.
Nowadays, an organic photosensitive body for a minus discharge is widely used as photosensitive body. Also, the environmental standard for generation of various kinds of gasses including ozone has become severer. Thus, a countermeasure is keenly demanded. The corona discharge method also has a problem in that the adhesion of nitrogenous compounds (NOx), etc., which are secondarily produced by ozone, to the surface of the photosensitive body causes an abnormal image.
Accordingly, an image forming apparatus adopting a contact-and-charge method has been developed as a substitute of the corona discharge method. This new type of image forming apparatus employs a charging member, such as a charging roller, to be brought into contact with the surface of the photosensitive body. According to the contact-and-charge method, the charging roller supplied with voltage is brought into contact with the surface of the photosensitive body. The surface of the photosensitive body is evenly charged by electric discharge through an air gap which, in the strict sense of the word, exists between the respective surfaces of the charging roller and photosensitive body. Therefore, according to the contact-and-charge method, the voltage to be applied to the surface of the photosensitive body can be lowered compared with that of the corona discharge method. As a result, advantageously, the amount of ozone to be produced is sharply reduced.
However, even in the contact-and-charge method, there is a situation in which the photosensitive body is not evenly charged because of difficulty in maintaining a required electric potential at a constant level. In other words, the unevenness or nonuniformity of the charged photosensitive body is caused by the reason why changes in circumstances for the use of the apparatus, such as a change in temperature of the charging roller or a change in temperature of the inside of the apparatus, (in the following, these are referred to as apparatus circumstantial changes) lead to changes in electrical characteristics, such as a resistance value or dielectric constant, of the charging roller.
In order to prevent the nonuniformity generated when charged, such a technique as disclosed in Japanese Laid-Open Patent Application No. Hei 4-186381 is proposed. According to this technique, the temperature of a charging roller is detected by a sensor and then the voltage to be applied to the charging roller is changed in accordance with the detected temperature.
However, since the dependence of the surface potential of the photosensitive body upon the temperature is not in proportion to the applied voltage of the charging roller, it is still difficult to completely prevent the nonuniformity.
In addition, another problem residues in that the electric potential of the charged photosensitive body fluctuates because of the abrasion of a photosensitive layer of the photosensitive body caused by friction. As a solution to this problem, a technique is known in which the amount of abrasion of the photosensitive layer is detected and, in accordance with the increase of the detected abrasion, the voltage to be applied to the charging roller is decreased, as disclosed in Japanese Laid-Open Patent Application No. Hei 5-27557.
However, under the influence of the abrasion of the photosensitive layer, the characteristic of the photosensitive body depends upon the temperature of the charging roller. That is, the characteristic presented when the charging roller has a high temperature is opposite to that presented when the charging roller has a low temperature. Therefore, a surface potential of the photosensitive body cannot be always maintained constant even if the voltage to be applied to the charging roller is decreased.
There is still another problem. Generally, if a charging roller is used as a charging member, the diameter of the charging roller is such smaller than that of a photosensitive body with the view of making the charging roller small in size. For this reason, even in a case in which a small quantity of toner which has little influence on an image has adhered to the charging roller via a cleaning device, the toner becomes an obtrusive stain by the frequent occurrence of the toner thereto. As a result, such a stain causes a defective part which is not sufficiently charged.
It is therefore a first object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality regardless of whether or not, when a voltage to be applied is corrected in accordance with apparatus circumstantial changes, a photosensitive body has a physical change suffered with the lapse of time.
It is a second object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality regardless of whether or not, when a voltage to be applied is corrected in accordance with a physical change of a photosensitive body suffered with the lapse of time, there are apparatus circumstantial changes.
It is a third object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality, when an OPC (organic photoconductive cell) type of photosensitive body is used, regardless of apparatus circumstantial changes or a physical change of the photosensitive body suffered with the lapse of time.
It is a fourth object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality when an OPC (organic photoconductive cell) type of photosensitive body is used.
It is a fifth object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality by always making a potential of a charged photosensitive body constant regardless of apparatus circumstantial changes or a physical change of the photosensitive body suffered with the lapse of time.
It is a sixth object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality without fluctuation of a potential of a charged photosensitive body even when a correction rule is changed.
It is a seventh object of the present invention to provide an image forming apparatus which is capable of forming an image of stable quality by reducing defective areas of a charged photosensitive body caused by adhesion of a toner to a charging member.
It is an eighth object of the present invention to provide an image forming apparatus which is capable of effectively preventing a charging member from being stained for circumstantial changes at a low cost without detecting apparatus circumstantial changes by means of a newly disposed mechanism.
It is a ninth object of the present invention to provide an image forming apparatus which is capable of preventing an image from becoming dark for the decrease of the sensitivity of a photosensitive body in a low temperature.
It is a tenth object of the present invention to provide an image forming apparatus which is capable of preventing the deterioration of an image caused by apparatus circumstantial changes.